Another thing you have to remember about putting music in videos is the copyright issue. I have seen plenty of videos posted by other people that had the audio completely disabled because Warner Music Group (WMG) or EMI didn't want their music used in the video without permission.

I have two videos with copyrighted music on them. They were sped up in Windows Movie Maker, which mutes the audio when you speed up the video. They are both recumbent rides on flat ground. The first one has "I'm So Excited" by The Pointer Sisters on it. That video is blocked in many countries due to copyright. Luckily the US isn't one of them.

The second one has a remix of "Hello Again" by The Cars. That video is not blocked anywhere, since WMG seems to have lightened up a bit about copyright.

Contrary to what TxDoc says, it is a PITA to find the right song to fit the mood of the video and to fit the video to the length of the song so the song isn't cut off. That's another reason I'm glad I don't have to put music on every video I make.

Here's the first video with "I'm So Excited": http://www.youtube.com/watch?v=3-MuzISJagE

Here's the second one with "Hello Again": http://www.youtube.com/watch?v=RsQNesMdvqs